NFS: Remove size hack in nfs_inode_attrs_need_update()

Prior to this patch, we used to always OK attribute updates that extended
the file size on the assumption that we might be performing writeback.
Now that we have attribute barriers to protect the writeback related updates,
we should remove this hack, as it can cause truncate() operations to
apparently be reverted if/when a readahead or getattr RPC call races
with our on-the-wire SETATTR.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-by: Chuck Lever <chuck.lever@oracle.com>